Processes contributing to the evolution and destruction of stratification in the Liverpool Bay ROFI

Howlett, Eleanor R..
2010
Processes contributing to the evolution and destruction of stratification in the Liverpool Bay ROFI.
Bangor University, School of Ocean Sciences, PhD Thesis.

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Abstract/Summary

Liverpool Bay is a ROFI (Region of Freshwater Influence), subject to semi-diurnal and enduring periods of stratification, controlled by a competition between lateral sources of buoyancy and mixing from tides, winds and waves. A simple 0D model, utilising developments in vertically resolving equations representing lateral buoyancy processes and wave mixing approximations, has been used in conjunction with a 2-year dataset from the Liverpool Bay Coastal Observatory. The Coastal Observatory dataset has provided new insight into the vertical current structure and wave climate of the region due to the availability of ADCP (Acoustic Doppler Current Profiler), HF radar and wave buoy data. Additionally, data from moorings has allowed for the examination of the temporal and vertical evolution of horizontal density gradients.
The model was used to examine four factors affecting the stratification of the Liverpool Bay ROFI: wave mixing, wind straining, the impact of depth- and temporally- varying horizontal density gradients, and the effect of advection and straining of the horizontal density gradient. Wind straining was found to be important, specifically in terms of enduring stratification; northerly winds induced a westward residual which pushed the front offshore, generating enduring stratification whilst a southerly wind induced an eastward residual which acted as a destructive mechanism. Advection, in addition to depth-mean straining, was found to be an important process affecting the stratification of Liverpool Bay; its phase, and thus influence, relative to the depth-mean straining contribution, was found to be a function of the vertical variation of the horizontal density gradient. Wave breaking was determined to be an important source of mixing; wave-induced bed stresses and thus the energy available for mixing from those stresses, however, became significan once the water column had already homogenised suggesting they are unimportant to the vertical structure of the Bay. Further work in the Liverpool Bay ROFI should consider the minor-axis component of both the currents and horizontal density gradients which may become important at certain times.